thumb|right|320px|The six flavors of quarks and their most likely decay modes. Mass decreases moving from right to left. A quark () is a generic type of physical particle that forms one of the two basic constituents of matter, the other being the lepton. Various species of quarks combine in specific ways to form protons and neutrons, in each case taking exactly three quarks to make the composite particle in question. There are six different types of quark, usually known as flavors: up, down, charm, strange, top, and bottom. (Their names were chosen arbitrarily based on the need to name them something that could be easily remembered and used.) The up and down varieties survive in profusion, and are distinguished by (among other things) their electric charge. It is this which makes the difference when quarks clump together to form protons or neutrons: a proton is made up of two up quarks and one down quark, yielding a net charge of +1;